The Japanese Maritime Self Defense Force flag is problematic. It's like Mississippi or Georgia's state flags before 2020, it's a call back to things that should be left in the history books.
To many Koreans (and likely Chinese) the wartime version of the rising sun flag is exactly the same as the Nazi flag if not worse. The anglosphere tends to gloss over the brutal behavior of Japan during this time, under this flag.
The difference with Germany is that they took responsibility and passed laws to regulate usage of the nazi flag, etc. Japan did not, and lied to the Korean government about things like the comfort women.
> were used explicitly for those hateful movements
Do you honestly believe the rising sun flag was not a part of a hateful movement? Is the fact that Korea was annexed and Koreans were treated awfully under this flag just a coincidence?
The swastika was a very popular symbol, and remains popular outside nazis. The swastika shouldn't (and really can't) be banned, but the Nazi usage of it should be.
The confederate flag had a bunch of variants and the popular version seen today was only used after as a symbol of hate. Should be banned.
The rising sun flag is just one variation of a couple themes used throughout Japanese history and it was used in a non-imperialist setting for hundreds of years since the Edo period. Its in the fuzzy place where most flags lie.
You can argue for the banning of that flag for those atrocities, but why stop there? Are you going to ignore the countless indigenous peoples who suffered under the American flag or all the colonialism done under the union jack? What about the Turkish flag which flew during the genocide they committed and still flies today. What you're essentially arguing for is banning all flags related to some kind of atrocity. That's most flags.
If you take a group of people and ask them what the rising sun flag represents, the majority response will be about WWII imperialist Japan. Which makes sense as they changed it immediately after their surrender in WWII. It legitimately is viewed like the nazi flag by the various Korean people I've talked to in Korea.
The context also matters. Korea has plenty of swastikas. Specifically at shrines/temples and on the ground; not being flown as part of flags.
"What about Y" is a weak argument to keep X flag. You could make that argument for basically any flag. To answer though, it's likely the US flag will be seen in a similar light once it's no longer a powerful hegemony. If the US had a flag that was only used during its "imperialist era" and a different modern one, one would have a stronger argument.
Should the UK retire the Union Jack because of what they did in India? The Nazi imagery is very closely associated with the specific group that committed the atrocities rather than Germany in general. The UK flag is less so. IMO, the rising sun flag is somewhere in-between, having been in use over a slightly more heterogenous time than the Nazi symbols, bit less so than the Union Jack.
>The Japanese Maritime Self Defense Force flag is problematic.
The flag of the National Republican Army is the Italian flag with a bird on it. Is the Italian flag also problematic due to its close relationship with the Axis?