We've been encountering the occasional (4 that we know of) issue with rules either throwing UnresolvedAssemblyError when needing to invoke a custom Rule-Utility-Function; or more recently not throwing the Error but resolving to an older version of the R-U-F (01-02-03) than one more recently introduced (01-02-27).
If you find a way to reproduce this issue, we here at Pega would like to know so we can address it. Some things that may help you reproduce it are to be aware of these things :
1) Does it occur when you make a new function ? Or when you make a new referring rule (such as a when-rule that uses the function) ?
2) Or does it occur only when you import a new function ?
3) Or maybe it only occurs if you import both a new function and a referring rule ?
4) Maybe it only occurs if you are importing a new function and a referring rule AND there's a coincident agent job, such as an SLA activity, that happens to kick in and use the referring rule or function WHILE you are importing it.
5) Does it occur only on a second node from the one you imported the rules into ? Or does it occur on the import node as well ?
6) Is your db server timezone set to the same one as your Pega server ?
7) Does the pulse offset-ms setting affect how often this occurs ?