At low frequencies (say up to 1MHz) an oscilloscope should be as accurate as it's DC specifications...
This is your opinion. This is not truth - it is tru
mpth.
Specifications tell frequency flatness is
+ 1dB from DC to 20MHz.
For draw acceptable 1kHz square what frequencies of sinewaves you need so that sum is what you see is "square wave" on this case.
Think that every these single sinewaves may have level error something between + or - 1dB. (including that also this DC may have 3% error. Now draw specifications acceptable error window example for this top part of this square wave. Is it inside this window.
You or who ever can define what is nice to have or how it should looks like what you like is ok but if it violate only this opinion it do not mean it violate given specifications.
And I do not mean that it should not be better. It should, of course.
When designing front end layout and topology it need look with signal eyes from dc to rf. It must not look with human eyes as we look if things on table are in good order. It need care more about how signal see it.
Besides that I have never seen this kind of behaviour on any oscilloscope I have used.
I have seen same situation also example in Owon XDS3000. And in my lifetime experience I have seen this situation many times.
Here Owon. Error is not so big but also there can see separate vertical bands have different input reactanse. (mostly capacitive) But owon input nominal capacitance is much lower and also changes are lower between bands.
Attacments just for example. (I do not have enough high voltage level gen for drive system with probe set 10x ) )
In images used 200MHz probes connected to Gen output where is also 50ohm extra feed through directly attached to gen output before probe adapter. 20M BW is on for reduce Owon noise and it do not affect this shape. (gen out imp is 50ohm and after then 50ohm feed thru and probe connected to this with probe bnc adapter)
Between images probe is untouched. You can see which image probe is compensated and then changed level and scope Vertical level band (Level band relay switch just between these levels) It can see with other level band probe is not anymore perfectly LF compensated.