Are they both spayed/neutered?
7 months is a bit old for a “baby bond”, but I think flemies tend to hit puberty later, so it’s possible the initial love-at-first site was due to Bertus not hitting puberty yet (if he hasn’t been neutered). Adults and babies tend to get along, but when puberty happens all of that can go out the window.
If that’s not the case, then it really is perplexing! Although I have heard of some similar cases (love at first sight and then fighting a few weeks later), likely due to the bonding process just being a bit rushed. Can you fill out the bonding template that’s pinned in the section (copy the template and then paste in your reply below)? That would help give us a bit more info and know what might have happened.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.