Cumulatively Count Gaps in Sequential Numbers Results in Different Answers When New Data Added

I asked a question a few days ago which you guys helped me solve and I am forever grateful! However, a new issue has presented itself and I am in need of your help once again!

Here's a link to the original problem: (R) Cumulatively Count Gaps in Sequential Numbers

I was trying to cumulatively counts gaps in sequential numbers for each selection of UniqueIDs. This was my dataset:

UniqueID  Month  
ABC123    1       
ABC123    2      
ABC123    3      
ABC123    4      
ABC123    6      
ABC123    7      
DEF456    3      
DEF456    4      
DEF456    10     
DEF456    11     
DEF456    12     
DEF456    14     
GHI789    2      
GHI789    3  
JKL012    12     
JKL012    13     
JKL012    14    

Using your help, I tweaked the code provided from the link above as follows:

data2=data %>%
       group_by(UniqueID) %>%
       mutate(Skip = if_else(Month - lag(Month, default = first(Month) - 1) - 1 > 0, 1, 0),
       CountSkip = cumsum(Skip))

data2 = data2%>% 
       group_by(UniqueID) %>%
       mutate(LastValue = if_else(Month == last(Month), 1, 0))

data2=as.data.frame(data2)
data2$FinalTally=ifelse(data2$LastValue==1 & data2$Month!=14,1,0)
data2$SeqCount=data2$FinalTally+data2$CountSkip

This was the resulting dataset:

UniqueID  Month  Skip CountSkip LastValue  FinalTally   SeqCount
ABC123    1      0    0         0          0            0
ABC123    2      0    0         0          0            0
ABC123    3      0    0         0          0            0 
ABC123    4      0    0         0          0            0
ABC123    6      1    1         0          0            1
ABC123    7      1    2         1          1            2
DEF456    3      0    0         0          0            0
DEF456    4      0    0         0          0            0
DEF456    10     1    1         0          0            1
DEF456    11     1    1         0          0            1
DEF456    12     1    1         0          0            1  
DEF456    14     2    2         1          0            2
GHI789    2      0    0         0          0            0
GHI789    3      0    1         1          1            1
JKL012    12     0    0         0          0            0
JKL012    13     0    0         0          0            0 
JKL012    14     0    0         1          0            0

This is what I wanted...or so I thought.

When adding in new data for the next month (15), I edited the second to last line of my code to account for 15 being the new final month. However, I noticed the sum of SeqCount by Month differed from the sum of that same month before the new data was added. I filtered down to one month and found an example of one UniqueID where the SeqCount sum had differed.

Here is an example before the new data was included:

UniqueID  Month  Skip CountSkip LastValue  FinalTally   SeqCount
ZZZ999    2      0    0         0          0            0
ZZZ999    3      0    0         0          0            0
ZZZ999    4      0    0         0          0            0 
ZZZ999    5      0    0         0          0            0
ZZZ999    6      0    0         1          1            1

Here is the example when the new data was included:

UniqueID  Month  Skip CountSkip LastValue  FinalTally   SeqCount
ZZZ999    2      0    0         0          0            0
ZZZ999    3      0    0         0          0            0
ZZZ999    4      0    0         0          0            0 
ZZZ999    5      0    0         0          0            0
ZZZ999    6      0    0         0          0            0
ZZZ999    15     1    1         1          0            1

This is the problem: Month 6 loses a value of SeqCount when new data is added in.

My ultimate goal is to run a regression model for each month with SeqCount as the response with some other columns as predictors (I didn't include them for ease of reading). Whenever I add new data in, the response will change and my estimates will not be consistent.

Is there a way I can structure my code differently so when I add new data, the logic of the code does not change the information from previous values of SeqCount?

Any help would be appreciated!

Thank you!