, Don't be shy of Dell EMC VXL team - they have some quality engineers whom I work with on a daily basis, fair enough many of them don't have as deep an insight into some areas of vSAN as my team (VMware vSAN GS) but that is expected since they are not working with just the vSAN aspect all day, every day and thus why we have things in place that they consult our team as backline when things get 'L3' (and this is the same as with HPE, Dell, Cisco, IBM, insert-vendor-that-also-sells-S&S-here).
This is not a design-decision relating to just the state your cluster is in right now - actually it won't allow changing size of anything when it is in a reduced-availability or otherwise 'unhealthy' state, I am not aware of when this originated (relatively sure it wasn't the case a few years ago) but I can understand the logic that making changes to something already in an impaired state (in the vast majority of cases anyway - what if the other node comes back and doesn't have space to accommodate the change?) probably isn't a great idea and focus should be on addressing the issue and getting data redundant/compliant again. Increasing size of Objects while their Storage Policy is being changed also will be denied.
Workaround for this is the same as if you had a necessity to create snapshots from (and thus with the same Storage Policy (SP) as the) vmdks with a dual-site mirroring policy - it is complaining because you are asking it to do an operation that requires 3 Fault Domains (e.g. siteA+siteB+Witness) for component placement but this is not available - telling it to try create/change the Object with that policy but manage with less if not possible (with the caveat that anything created in this manner won't have the same redundancy as the parent Object) by applying a policy with Force-Provisioning(FP) on it (e.g. try make the Object/apply the change as FTT=1 but if not possible make it as FTT=0).
I assumed this was the case but did test this just now to be sure - it looks like you actually have to apply the FP=1 policy on it to make the change (which makes sense as you are changing the Object itself not making an Object from it like a snapshot) as opposed to just changing the policy rule so if doing this I would advise just cloning the SP in use, adding FP=1 rule to it and applying this only to the vmdks you need to expand (as opposed to applying an FP=1 SP to all the Objects using the original SP), do ensure to revert this change and re-apply the original SP once things are back to normal.
By the by, just an FYI - I would advise if you have questions like this to just post it as a new topic, if it isn't 100% what the original thread was discussing it makes things convoluted.