The R7000 shifters are 11-speed shifters and require 11-speed pull ratio rear deraileur and 11-speed cassette.
I'm not sure if your Tiagra is 4700 or earlier. If it's 4700 then despite being a 10-speed system it uses the 11-speed derailleur cable pull ratio, so it will work with 11-speed shifters provided that you use 11-speed cassette.
If the Tiagra is ...
I suspect you have cables and/or housings that are either old or off-brand. The friction in these can be surprisingly large. Thus the issue can be cable friction like Michael suggests.
Buy new Shimano 1.2mm stainless steel inner cable. If you have a recent system with a ridiculously high sprocket count, you may benefit from an Optislick ...
I think it’s most likely that your cable has excessive friction, especially in the last part where it makes a tight bend from the frame into the rear derailleur.
If you disconnect the cable from the derailleur (i.e. open the clamping screw) I assume it works as expected?
Does this also occur on the big chainring? Otherwise it might also be a too long chain.