It has been a common occurrence with a bug for the past 1.5 years, random despawning of bases due to decay issues or some placeables specifically.
Although you have not confirmed or clarified fully, it would appear you do not incorporate the required number of pieces in order to achieve a full decay timer.
Is it 20 pieces altogether? Is the decay timer currently 480 hours? If not, then you will have a lower timer. Then your buildings will decay faster.
What does it say in your logs? Are there entries that state that they certain pieces were abandoned and will decay, then does it showed that they decayed in those logs?
If you see that type of stuff then it isn’t a bug, it is intended.