User can't see some emails after loading PST

The PST is probably complete because it shows messages through the export date in the test account; the missing mail and stopped delivery instead point to how the file or Outlook profile was attached. Keep the Classic Outlook profile and make a copy of the PST. In File > Open & Export, select Open Outlook Data File, expand the PST's top-level folder, check each subfolder, and set search scope to All Mailboxes. If the full 2020–present range appears, move mail into the new mailbox in folder batches and confirm each batch syncs before continuing. Do not use new Outlook for a bulk import: it can open and move PST email, but Microsoft does not currently support bulk PST import there. If Classic Outlook still omits items, close Outlook and run SCANPST.EXE against the copy. Since the same PST opens elsewhere, avoid re-exporting until these profile and repair checks are finished.