Editing cell value changes its formatting

Question

1. Enter the value 18/04/2020 08:10:00 in a sell.
2. Set the formatting of the sell to Hour(1):Minute(01)
   • Actual value 18/04/2020 08:10:00
   • Display value: 8:10
3. Edit the value of the sell to become 18/04/2020 08:11:00
   • Actual value 18/04/2020 08:11:00
   • Expected Display value: 8:11
   • Expected Formatting: Hour(1):Minute(01) (as set earlier)
   • Current Display value: 18/04/2020 08:11:00
   • Current Formatting: Day(05)/Month(08)/Year(1930) Hour(01):Minute(01):Second(01)

Answer 1

You said:

1. Formatting for Column A is set to: Hour(1):Minute(01)
2. The values in that column are like: 1054496:09 (Timestamp with date)

I am not able to reproduce your formatting.
If one uses Hour(1):Minute(01) to change the timestamp 16/04/2020 08:09:00 to 1054496:09, Google sheets automatically converts the cell to 8:09 while inside the cell one reads 16/04/2020 08:09:00 or 04/16/2020 08:09:00 (depending on the locale of the sheet).
In any case...

Solution

1. Copy the value of any of your converted timestamps in the format 1054398:09 and paste it in an unformatted cell. The value in the cell will be 1054398:09:00.000 , while it will be shown as 1054398:09. Do NOT manually format the cell.

2. Use the Paint format (Roller) tool from the menu and format the other cells. Do NOT manually format the cells.

These formatted cells will stay in the format 1054398:09 no matter the changes.

+---------------------+-------------------------------------+
| 18/04/2020 08:10:00 | Formatted as Timestamp/Date time    |
+---------------------+-------------------------------------+
| 8:10                | Formatted as Hour(1):Minute(01)     |
+---------------------+-------------------------------------+
| 1054544:10:00       | UNFORMATTED/Formatted as "nothing"  |
+---------------------+-------------------------------------+