Didnt at least some engineers transfer after acquisition?

Maybe. I don't think a lot did. Amstrad did not acquire the company: it bought some rights to use the Sinclair Research brand.

That's why the next computer Sir Clive launched was the Cambridge Computers Z88. But note, some of the later bicycles were Sinclair Research branded:

https://en.wikipedia.org/wiki/Sinclair_Zike

https://en.wikipedia.org/wiki/A-bike

Amstrad did not acquire or develop the Sinclair QL, for instance, but it did sell Sinclair-branded x86 PCs.

https://www.computinghistory.org.uk/det/3404/sinclair-pc200/

It sold on some stock of existing ZX Spectrum hardware, but mostly it sold models it developed:

* Spectrum +2 -- a Spectrum 128 with a mechanical keyboard and built-in cassette drive

* Spectrum +3 -- a redesigned Spectrum 128 with a DOS from Locomotive Software and a 3" (not 3.5") floppy drive. Dropped compatibility with 48 peripherals such as Interface 1 and Microdrives, and 128 peripherals such as the numeric keypad, serial ports, etc. Added the ability to page out the ROM and replace it with RAM, so it could run CP/M 3, also ported by Locomotive.

* Spectrum +2A, the black +2: a cut-down +3 with a cassette drive.

These were designed by Amstrad engineers and contractors, and manufactured by Amstrad. No Sinclair involvement I'm aware of at all.