Rich-Text formatting is not an OPT in/out / turn on/off feature. It's always turned on. To use it:
Put the cell in edit mode
Type the text
Select the text part to apply bold, italics, color
Apply the desired format by using the Google Sheets toolbar or keyboard-shortcuts like Ctrl+b for bold, Ctrl+i for italics, etc.
Put the cell in display mode (press ...
The same occurs in Excel Ref Excel number format to only show decimals when necessary, unfortunatelly the conditional formatting options in Google Sheets don't include numbering formatting.
What about this simple workaround?
Set number formatting to Automatic
Add and empty string to the result of the original formula:
If your number is 3.14159265 you will get 3.14159265, if you want less there is no way to have 3.14 and have no ".00" in whole numbers.
You cannot achieve it with no auxiliary column where you use trunk(A2,2) and then use automatic in the format for the result
I just changed the format of column J in your example file to "automatic" and numbers are shown as ...
d/m/Y is an ambiguous date formatting.
It's very likely that this occurs because the spreadsheet regional setting used in the spreadsheet doesn't support d/m/Y as a date formatting. Please note that the screenshot included in the question shows column J values having a "day" value greater than 12, with exception of 11/01/2019 have the a white cell ...
Using 'Automatic' or 'Plain Text' format usually does the behavior you are describing. If you try to format a number that way you will end up with a . after a whole number so pretty much you need to have a helper column. The closest you can get with using only formatting is to do 0.0### which will always show 1 decimal place but expand out 4 decimal places ...