APM Server version (apm-server version): 9.0

Description of the problem including expected versus actual behavior:

we've observed lsmsize going negative which doesn't make much sense

Steps to reproduce:

Please include a minimal but complete recreation of the problem,
including server configuration, agent(s) used, etc. The easier you make it
for us to reproduce it, the more likely that somebody will take the time to
look at it.

Provide logs (if relevant):