INSERTING ROWS and LOSING FORMAT

I own a spreadsheet that has daily data inputted for, say 7, years; to add
rows to increase my size, I "Insert" rows at the top of the spreadsheet, say
10 rows every day! And, for years, only until recently, every new cell was
properly formatted.

Now, each time I "Insert" a rows, my column formatted (as todays time) as
follows:

12:43am becomes 12:43, or
1:27pm becomes 1:27

and, this occurs EVERY DAY for 3 rows only of my newly "INSERTED" 12 to 14
rows!

I have never had this happen before, and I immediately fix the dilemma with
the Format Painter -- just one extra step that I never, before, had to
perform!

CAN ANYONE INFORM ME AS TO WHY I, SUDDENLY, INCURRED, THIS PROBLEM?

Thank you.

FLKulchar