I’ve been using Code Stream Custom Integrations to refactor Sam McGeown’s Image as Code pipeline using Python 3.
Occasionally I would get error stating “The saved Custom Integration version is no longer available”
One of the issues I ran into a while ago was the apparent loss of a Custom Integration version in a pipeline.

What the heck? This particular Custom Integration hasn’t been touched for over two months.

The exported Pipeline has the correct version.

However the Pipeline designer doesn’t see it.

The work around is to delete some old versions/integrations, save a new version of the Custom Integration, then update the pipeline. Not ideal, but it is what it is.
A GSS case was opened at the end of December 2020, which eventually led to discussing the issue with some developers. After some research they came back and said it was a database configuration which limited me to 300 Custom Integrations and Versions. Hopefully they’ll get it fixed in the near future.
In the mean time, at least I know what is happening.
Happy coding.