Welcome, Tantalite, to Apple Support Communities!
You can perform the definitive test presented within my message at Email photo as attachment, not embedded, … - Apple Community (checking against the standards, that I linked to, there).
Since the definitive test demonstrates that Apple Mail generates standards compliant emails (specifically following the MIME standards), the only explanation for a very small subset of email clients failing to “do the right thing”™️, with such received emails, is a failure to properly handle such standards compliant emails.
(Actually. Technically speaking. Such misbehavior, by an email client, in not “doing the right thing”™️, with standards compliant received emails, is an allowed behavior. According to the International Internet Standards.
The standards do not require any specific behaviors of such receiving email clients.
Instead, it is up to the users of such clients to determine whether such behavior is acceptable, or not.)
Unfortunately, the email clients known to fall in that very small subset of email clients, have not had substantive updates or upgrades since about 2010, or before.
(Additionally, the company responsible for that very small subset of email clients is all too well known—one could say notorious—for “flaunting” international internet standards.)
As for the assertion that “everything worked fine [prior] to version 14.1”: you appear to be unaware of the many times, in prior versions of iOS, where similar “breakage” of Outlook occurred, over many years.
This is, actually, a very old “battle”.