Hey,
I investigated this during the Angel compatibility work and so far I cannot reproduce a general issue with Angel ruins not spawning.
In my playtests with angelsrefining, angelspetrochem and angelssmelting active, all 15 Angel-related ruins were registered correctly. I also found Angel structures in normal gameplay and have a screenshot of an Angel facility ruin that spawned naturally.
With the default Abandoned Ruins spawn settings and the current Combined weighting, the tested Angel stack has roughly a 0.40% chance per eligible newly generated chunk to select an Angel ruin. That is about 6.2% of all ruin selections, so they are intentionally not especially common.
For a quick test, you can temporarily increase the ruin spawn rate in the mod settings to an excessively high value and explore newly generated terrain. I would recommend doing this in a new test game, though, so you do not unnecessarily affect your normal save.
Could you also post your full mod list or, preferably, your factorio-current.log after loading the save?
There may be another mod in your stack affecting the available Angel prototypes or the ruin pool, and the log would make that much easier to identify.