Blu-ray movies with Dolby Atmos or DTS:X soundtrack, on receivers with no support?

[zak88lx]

My current AVR for watching Blu-ray movies is an older Yamaha RX-V3800.
I absolutely love this receiver, and it can handle pretty much any DVD or Blu-ray soundtrack that I have thrown at it (see decoder list below).

-DTS 96/24
-DTS Neo:6
-Dolby TrueHD
-DTS decoder
-DTS-ES Discrete 6.1
-DTS-HD decoder
-Dolby Digital
-Dolby Digital Plus
-Dolby Pro Logic
-Dolby Pro Logic II
-Dolby Pro Logic IIx

Over Christmas I received "The Mummy - 2017" on Blu-ray, and I noticed on the case that it only offered a single English soundtrack: "Dolby Atmos"!
I popped the disc into my player, and the on screen Setup menu confirmed what the case specified, and I only had the following options:

-English: Dolby Atmos
-Spanish: DTS 5.1
-French: DTS 5.1

I selected the English: Dolby Atmos soundtrack, and was concerned at first that I might not hear any audio, but I did, and my Yamaha displayed "Dolby TrueHD 7.1, and the audio sounded fine.

Questions:
-Is this normal behaviour for an AVR that has no Dolby Atmos support?

-Is Dolby TrueHD the automatic replacement if your receiver can't handle Dolby Atmos?

-Is my receiver trying to convert this Dolby Atmos soundtrack to Dolby TrueHD, or does the disc actually contain a hidden TrueHD soundtrack?

-What would happen with a DTS:X only soundtrack?

Sorry for all the questions.

Thanks!