30+ year user of Office applications going back to pre-Outlook days when MS first bundled Word & Excel to compete against WordPerfect and Lotus 1-2-3.
Also a MS Partner that works with lots of small orgs including non-profits.
Formatting -- fonts, colors, styles, themes -- bites everyone including me still.
As Stefan Blom points out Copy/Paste issues are separate from MS changing default font from Calibri to Aptos in Office Theme.thmx.
Instead it is part of the myriad of settings and options for Copy/Cut/Paste and is different in different applications.
I just created a blank Word docx and when I go to paste text from here into the document via Home > Paste drop-down there are 5 paste options using each of the "clipboard" icons:"

Which are (left to right):
- Use Destination Theme (H)
- Keep Source Formatting (K)
- Merge Formatting (M)
- Picture (U)
- Keep Text Only (T)
*AND*
Those options change depending on the top item in the Clipboard *AND* the source and destination of the item being pasted!
(e.g. if the first item on the Clipboard is an image then the only option is Picture.)
(**Remember that Win+V opens the Clipboard contents dialog)
The default Paste option (via Ctrl+V, context menu, etc.) is set via the "Set Default Paste ..." drop-down menu option (also available from Ribbon File > Options > Advanced) includes the "Cut, copy and paste" options (may have to scroll down to see all of them):
<insert pic of person with hair on fire>
The 3rd one -- "Pasting between documents when style definitions conflict:" may be getting Caroline117 above.
Note that it mentions Style -- not Theme.
Also note the last item "Use smart cut and paste" and the "Settings..." button which opens this:
<insert pic of person with hair on fire running down street naked screaming>
And this is all just in Word ...
I have not verified but Outlook, OneNote ... will handle these options differently
On a positive note, Word, Outlook and OneNote still seem to be handling Ctrl+V similarly and offering a Paste Options button with drop-down (see settting above) at end of pasted content:
Clicking the drop-down offers the same Paste Options as using the drop-down from Ribbon menus.
Ideally, in a larger org or department such settings, fonts, colors, styles, themes ... would all be standardized and implemented via unique Office Theme.thmx, template and setting files (differnt for each application Excel .XLTX in XLStart, Word .DOTX ...) either stored on a server or distributed with each installation of Office within the org/department.
With software that has been around this long there are countless options/features added over the many years ...
"With every wish there comes a curse"