Re-highlighting sometimes fails with clangd

After doing some editing, the highlighting procedure is sometimes not re-run, resulting e.g. in new code staying un-highlighted. Sometimes only new code is affected, sometimes the entire file loses its existing highlighting as well. Example log output for an occurrence of the first case:

qtc.clangcodemodel.clangd.timing: highlighting: starting
qtc.clangcodemodel.clangd.timing: highlighting: subtask started at Fri Nov 26 12:22:01 2021
qtc.clangcodemodel.clangd.highlight: LSP tokens outdated; aborting highlighting procedure 60 62
qtc.clangcodemodel.clangd.timing: highlighting: subtask stopped at Fri Nov 26 12:22:01 2021
qtc.clangcodemodel.clangd.timing: highlighting: subtask started at Fri Nov 26 12:22:04 2021
qtc.clangcodemodel.clangd.highlight: LSP tokens outdated; aborting highlighting procedure 66 67
qtc.clangcodemodel.clangd.timing: highlighting: subtask stopped at Fri Nov 26 12:22:04 2021

Either there's something off with our versioning, or we are failing to request new highlighting, or clangd does not send it.
For the example above, a timestamp comparison with the language inspector seems to indicate that a new semanticTokens message was received after the quoted log output, but if that is so, then it was apparently not forwarded to the clangd-specific client code.