From my little experience with surround systems, having the rear speaker relatively quiet is quite common. Maybe someone with more experience then me can contradict, but I rembember the same problem on old Creative Labs 4.0 and 7.1 systems with 5.1 contents and also a friend of mine with a 5.1 system playing DVDs told me the same thing. I think there are several reasons for this.
First of all, in front of you you have not only the left and right speakers, but also the center speaker, where dialogs are located: they are loud, of course, because dialogues must be well defined, so in general you'll hear more sound coming from the front than from the rear because of the additional speaker and its purpose.
Secondly, the feeling is that movie makers are using rear speakers sparingly, probably as a choice. It may also depend on the soundtrack language in use: here in Italy, dubbed soundtracks are often lower quality with regards to multitrack audio, compared to the original English soundtrack.
Lastly, and this is my own idea (probably wrong?), I think that Samsung took the 5.1 setup quite literally: as also Dolby site confirms (https://www.dolby.com/us/en/speaker-setup-guides/index.html), in a 5.1 setup surround speakers are to be placed more like side speakers rather than back speakers, while in 7.1 and Atmos setups, rear speakers are really on the back. If I understand it correctly, in the HW-Q90R the side speakers are indeed installed on the front soundbar and their output is meant to be reflected on the side walls to appear to come from your sides: this could explain why rear speakers are not very loud even when 5.1 surround sound is played: they are probably outputting just a fraction of the surround sound, most of which is played by the side speakers.
Having side speakers output actually coming from the front perhaps also increases the volume of the front contents even more, compared to the back ones (see previous point).
I tested my HW-Q90R with my good old LG BD390 Bluray player, which has a good audio test feature, and used a SPL app on my phone (plus my own ears) to check whether sound levels are ok. The Bluray player is set up to 7.1 output, so I can check each speaker individually (except for the top ones, this player is not Atmos enabled). Indeed, side speakers do play from the front unit (more apart then the left and right front speakers), while rear speakers are working as intended, having their default volume on par with front speakers. Indeed, to calibrate the whole system, I just had to lower side speaker volume to -2, keeping front L&R, C and rear L&R to 0.
So, my conclusion is that rear speakers (at least in my case) are working properly and their potential volume is fine even at the default settings, it's just that for 5.1 contents rear speakers are used sparingly, both because of the contents and probably also because of the way in which Samsung has laid out 5.1 audio on this 7.1.4 system. Which could be logical.
Important note: all these tests/considerations are being made with "Standard" sound mode, which leaves multritrack contents intact, without any upmixing.
What do other think?
