Generally it is not recommended mixture of both, nevertheless if it work on your system then no need to worry too much.
Since you are using Asus motherboard however reference to the MSI screenshot will results in confusion for the system specification although both are using same Z390 chipset.
the following is the correct specification of your motherboard memory:
View attachment 38441
Your Asus motherboard can support:
1) Capacity = 128GB Max = 4 x DDR4 memory slots
2) Speed = 4266Mhz (o.c.) Max . *per above screenshot any speed at 2800Mhz & beyond is considered as memory overclocking.
Basically your motherboard are designed with 4 memory slots split into 2 Channels (A) & (B) with "dual rank" per channel:
1) Channel A (dual rank) = Slot "A1" (32GB max) + Slot "A2" (32GB max) = Max 64GB
2) Channel B (dual rank) = Slot "B1" (32GB max) + Slot "B2" (32GB max) = Max 64GB
any RAM channel exeed the dual rank per channel does not bring any further advantages to the performance in general, which is why many suggest configuration 1) below:
1) 2 stick MEMORY with dual rank put in slot A2 & B2 is easier to run XMP speed example beyond 3200Mhz, compared to
2)
4 stick MEMORY with dual rank put in
slot "A1+A2" &
"B1+B2" is
harder to run XMP example beyond 3200Mhz with possibility not able to do XMP at all
However if you have 4 stick single rank memory tabulated into 4 memory slots, it will work like configuration 1) of dual rank memory per channel with XMP speed, yet it will depends on the cpu, motherboard, memory quality.
Hope this quick summary help to clear some of your doubt.
I am sure there are many others will have better explanation than me if you need more information on memory configuration.