Asked this question to Mathworks support and got a reply.
It is a known bug which has been fixed in version 2021b. So the workarounds/solution I was give are as follows:
1) Upgrade to the latest version of MATLAB. The issue is not present in MATLAB R2021b and later.
2) Input the symbols you need in the Test Sequence Editor by pasting them from the clipboard. You can view multiple objects in the clipboard by pressing "Windows + V". You can then select the object you wish to paste using the mouse.
3) Edit the Test Sequence block programmatically from the MATLAB Command Window. For an example of this workflow, refer to the documentation links below:
https://se.mathworks.com/help/releases/R2021a/sltest/ug/programmatically-create-a-test-sequence.html
Hope this helps someone else.