Barely any IRS offices are getting caught up at this point. Those checks are likely still waiting for the mail to be opened.

Where do the checks go?  IRS doesn't cash many checks itself, it directs mail to private lockboxes operated by banks whose employees are still working.  IRS did shut down some PO boxes, I think in Hartford, CT, but not until July 1 and that was announced months ago.  

Five checks for five separate returns, but in the same envelope, may have confused someone enough that they were set aside and misplaced.  

Here is the latest from the IRS:

https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue

Apparently IRS has kept the 1041 check-cashing operation at its own Service Centers.  That may be because most of the returns are filed by banks and other corporate fiduciaries that make online payments.

IRS used to have machines that scanned unopened envelopes to find those with checks enclosed -- the magnetic stripes would be enough to sort them out.  They probably still do that, but they also have machines that scan larger envelopes, like the ones you might use to send five checks and vouchers, for explosives or other harmful chemicals.  Maybe that's where the bottleneck lies here.