@scylla gets it right.
But there is, of course, the PR Residency Obligation. Why a PR remains outside Canada is totally NOT relevant in determining whether the PR has met the RO. A PR can remain outside Canada for up to 1095 days within the first five years after landing, NO QUESTIONS asked. And, as long as on any day after the fifth year anniversary of landing the PR has been present in Canada for at least 730 days within the five years immediately preceding that day, the PR is in compliance with the RO, no explanation necessary.
There is no preemptive procedure in anticipation of a future Residency Obligation breach. No point in sending IRCC an explanatory letter upfront.
Any PR is subject to a RO compliance determination when coming to Canada, upon arrival at a PoE. Most times PRs are waived through without being questioned much about RO compliance. What triggers an RO compliance examination at the PoE is a big subject addressed at length and in-depth in many topics here. If the returning PR is questioned about RO compliance, and is not in compliance, THEN the PR can and should offer an explanation. It is quite likely that CBSA will be at least somewhat more flexible if not outright lenient in this regard for SOME (not all) PRs asserting there would not have been a breach of the RO except for this year's covid-19 crisis. No guarantees however.
