that does sound very strange! and i complete understand what you mean about the adobe subscription model, in fact it was because of that very reason i opted for the Affinity range 2 years ago! - update, after much tedious testing for half the day today i discovered what the issue was, it was a combination of two things, 1) somehow and for whatever reason the 'depth of field' live filter layer was tripping the project out! every time i would 'merge visible' it would crash, funny that was the first time i actually used that tool, 2). testing on another project (minus the 'depth of field). it would crash every time i would send a pixel layer via plugin to filter forge - it would crash, easy solution = i'll only use filter forge in standalone mode as it is not worth the hassle, besides it is not that hard to export from AP and import into FF and vice verse.
Perhaps it would be worth gathering up as much info as you can and email the Affinity guys directly, because it is definitely an issue that can be resolved, i say this as i have used the apps pretty much all day every day for 2 years and have never had issues (except for the past two days 😃)
Let me know how you go, don't succumb to Adobe... & good luck!