Unexpected naming of chunks after manualChunks function

[oatymart]

• Rollup Version: 2.38.5
• Operating System (or Browser): OSX 10.15.6
• Node Version (if applicable): 10.23.0
• Link to reproduction: https://repl.it/@oatymart/rollup-repro (REPL is slightly contrived, since I wouldn't normally use switch in manualChunks or mix and match chunk sizes. However I discovered the inconsistency while investigating why my single-module chunks would never take their specified names.)

Description

Given a manualChunks function for code-splitting based on language files, similar to that from the documentation (https://rollupjs.org/guide/en/#outputmanualchunks).
The resulting chunk naming is inconsistent.

Expected Behavior

Any chunk created by a manualChunks return value should have a name matching the string returned.

Actual Behavior

Chunks containing multiple modules take the name string returned from manualChunks function.
Chunks containing only 1 module take a name derived from the module filename.

[akauppi]

My project's also affected. It's not a huge turn-off but would definitely reduce the feeling of overwhelmedness often associated with packaging.

[lukastaegert]

This is happening if there is a single dynamic entry point (i.e. target of a dynamic import(...)) in the chunk.
The issue was caused by a conflict with the name derived from the dynamic entry point with the manual chunk name. #4040 will switch the naming priorities here to avoid this and prefer the manual chunk name.