Its really hard to tell from a technical perspective, especially without having closely monitored all of your digital activity (and those that you have been in close contact with) in the days/weeks leading up receiving the ads. Some things that Meta could have done (in varying degrees of realism) include:

• read anything you downloaded from your Matrix client, like file attachments
• read your notifications if they contain any contents of the conversation
• read your clipboard if you copy/pasted anything into/out of a Matrix client
• actively participating in the room and associated your Matrix ID to your Meta account(s)
• scraped the contents of the room if it is public and unencrypted
• others in the Matrix room saved your Matrix ID in your contact information within their contacts
• Meta is recording your screen outside of Meta's apps
• a Meta library is used in another app/service on your device that is sharing information back to Meta
• read an attachment that you downloaded elsewhere then shared on Matrix
• Meta read screenshots you or others took of the conversation
• Meta has a back door in the Matrix server or client software used
• the administrators of your Matrix home server (or the administrors of any other home server in the room) are sharing non-encrypted information to Meta to offset hosting costs
• Meta is running a home server of a user in the room
• you or someone you are associated with clicked on a link shared in the Matrix room that contained a tracker or led to a site that contained a tracker

Its really hard to comprehensively and conclusively avoid all "spying" that Meta/Instagram could do to you. The best thing that you could do is something that many people aren't capable or willing to do - not install any Meta software, don't use any Meta services, block any Meta IP addresses and/or domain names, and advocate that those around you do the same.

Realistically, the best advice that youre going to get has already been said. Use the web browser instead of the app as much as possible, ideally in a different browser and/or user profile. If you must have the app installed, keep it in a separate profile and kill the app and/or profile whenever it is not in use. Review all of your security and privacy settings in all Meta apps. Review any apps/services you allowed Meta to connect to/from (and the security/privacy settings of those apps). Reduce the amount of information that you enter/share on Meta platforms. Review the other users that you are connected with on Meta's platforms.