Good to hear you got it sorted. I can recall both Giants I owned having that same spacer on the drive side too, although they were 2x10 setups with 11-36 cassettes, so chain drop in reverse wasn't an issue anyway.
I'm assuming your new bike is boost, with Shimano boost cranks? If so, the chain ring will already be 3mm further out, and the BB spacer adds another 2.5mm. Clearly too much offset for the extended range cassette.
The reason for the spacer is some frames have a 92mm wide shell, others have a 89.5mm shell (why? who knows), and the cranks are designed for a total bottom bracket assembled width of 96mm (92mm shell + 2mm flange on each bearing housing). Therefore a 89.5mm shell needs a spacer to fill the remaining 2.5mm, otherwise the cranks will float laterally. Traditionally the spacer has been fitted to the drive side in something of a throwback to threaded bottom brackets with outboard bearings which catered for the possibility of fitting a BB-mounted front derailleur or chainguide, so always had a spacer on the driveside in the absence of such an attachment. Given that a press-fit shell is too wide for such an attachment, it's less relevant which side the spacer, if required, goes on.
Boost spacing doesn't really come into play here. The extra 3mm offset at the chainring is to match the 3mm outboard offset of the cassette compared to a Q/R or 142mm T/A rear end, so the relative chainline remains the same, the whole lot is just shifted 3mm to the right. Coming back to a point I made earlier in this thread, the 11-sp. cassette is wider than 8,9 & 10-sp, with the extra width fiitted inboard. That results in greater chain deflection on low gear compared to an 8, 9 or 10-sp. drivetrain.