so I was playing around with some custom synthetic races and I kept having explosive population decline, 1 a turn on my best planets, I looked around and their was another thread I couldn't find in recent threads about it reporting it vaguely bk in like may and a few times since

so i decided to isolate what was causing it
it doesn't seem to be linked to world types its happened on worlds with - growth and without its happened on capital and regular worlds precursor worlds ect
it doesn't seem to mater how much housing you have (some people where suggesting high level housing upgrades or any housing upgrades might be causing it)
I don't think its pollution as some have indicated it might be over 50 % pollution as I've had it happen on planets with 14% 41% extra
but what i found out was (apart from fact that making a fleet of 28 synthetics colony ships takes some time) , REGUARDLESS of housing existence or level or world capacity, PLACE 28 pop on planet as a synthetic civilization & the population would start to crash to 1, if I pulled them all off it stayed 1, if I made more or if I placed them bk from orbit it started falling again at one a turn till it was back to 1
(I tend to use a similar tech path & ideology bonuses[but not exact] each time so I'm not sure if theirs a interaction there)

the only way U have found to FIX it so far was i built a shrine on my capital creating 2 organics, then I moved my last synthetics off world, saved and restarted game(not sure if that was necessary, didn't seem to change it any other time I tried it)
it would then let me increase population normally regardless of housing level, till I hit 28 and then bug kicked in again



In conclusion at the moment and seemingly since may this year SYNTHETICS have a HIDDEN POP CAP OF 27

if you can find a way of getting past 27 without catastrophic population decline or a better/more repeatable/easier fix for broken world id be interested to know it