HUGE fonts in Email Body and Sign Off

My colleagues and I have been experiencing emails that look completely fine to us when we send out, but when we receive replies (or see the emails we have been CC-ed in, to each other), random parts of the email signatures and email body get extremely huge and disproportionate. It even extends to random table lines where some parts of it end up in Bold and are think, while other parts are ok. Does anyone have an idea of what's going on? My colleagues macOS are the latest versions (except mine - running on Catalina still) and both are using latest updated versions of Microsoft365, but predominantly only one of them seem to be sending out most emails w the disjointed fonts.. it's just not a good look for us especially when clients and vendors are receiving emails like that. We're not sure if this is an Outlook issue or a compatibility issue or something else completely. Looking for any answers that can potentially save us from this!

Attaching some screenshots here to show what I mean: