No, it's not necessarily indicative of cheating. It's been widely expected that mail-in votes would be extremely lopsided in favor of the Dems, regardless of state/district and that in person votes would be lopsided in favor of the GOP (again, regardless of state/district) because of how each electorate views the voting process during the pandemic. Dems were more likely to mail in their ballot, out of fears of COVID, and GOP voters were more likely to vote in person out of fears that their mail-in vote would be tossed out or conveniently "lost".
What you're seeing is mail-in votes (already expected to heavily favor Dems) coming in from Dem-heavy districts (large cities, etc.) in states Biden was already projected to be competitive in, in the first place. That all adds up to Biden overtaking Trump in these states. It looks fucky because the votes are coming in later, but that's because a lot of these states have provisions to count in-person ballots first and because the USPS is still delivering ballots mailed over the past week or so.