I see it as antagonistic by local vehicle crews, particularly those who look up to the myth of the old Soviet Empire.
Bit like you might see in other countries where they might fly the old flags of past era (Confederate flag comes to mind, but only because I can't think of others off the top of my head).
As other might say it is a morale booster.
Nothing more than that and given Russia's official voice (Russian Embassy on Twitter until very recently for example) I would think that they see it as not a negative to promote it.
The red flag on the tank was seen on February 25 in the push towards Kherson, and has been repeated many times.
And the other vehicles (APCs and trucks) flying it are from around 3-4 days ago.
Considering how hard the propaganda machine at home on Russian soil is working, a flying Soviet flag is like flipping the bird at the rest of the (sanction-flinging) world.
Also, ever since the Soviet Union collapse I see the old Soviet flag on a fairly frequent basis, usually with old timers who have good memories (or old Soviet patriots) and with new young Russians too.
It reminds me of the campaign to bring back the old Soviet anthem, and the old flag was never far behind...
So, tbh, its nothing new and I was expecting it..
(See other tank formations flying Orthodox flags, and of course the Novorossiya/New Russia confederation battle flags too)
Additional side note, related to the above:
Following Putin's statement in an interview in 2014 that the territories of Kharkiv, Luhansk, Donetsk, Kherson, Mykolaiv and Odessa were part of what was called Novorossiya (New Russia), self-proclaimed DPR and LPR proclaimed the confederation of Novorossiya and their desire to enlarge this across southern Ukraine.
The above image appeared in 2015 during DPR and LPR exercises.
Maybe there needs to be more context given to the source of the Russian Defense video. What if they are not Russian troops but rather DPR/LPR troops instead. many would lump them together but I think the differences are important.
Additional:
Given that it seems (without confirming) that much of the footage is of only a few vehicles and seems to be all from the south of Ukraine (for example the tank at Kherson dominates a lot of the reports) it is likely that the footage is of DPR / LPR forces or at least Russian sympathizers / supporters of them (more likely) - the Soviet flag has been seen most often in these parts:
Donbass and Donetsk:
Not sure if this is a great source but should show the romanticism for past Soviet days in these southern regions:
https://www.thedailybeast.com/pro-russian-protestors-in-ukraine-dream-of-soviet-glory-days
For many of the people here the Berlin Wall never came down, or at least it shouldn’t have done. .. At pro-Russian demonstrations ... in different cities the modern day Russian flag, while popular, always takes a clear second place to the flags of the USSR and of the Russian Empire before it. The yearning for a vague idea of “glory” that will come with being part of a greater Russian “whole” runs through everything.
https://www.ft.com/content/1425647c-297f-11e6-8ba3-cdd781d02d89
Brandishing a chest pin of Joseph Stalin, Eduard Basurin, deputy defence minister for the local pro-Russian militants, defends the former Soviet leader for raising the “USSR to a level when it was respected in the entire world”