Is imprisonment abroad a reasonable excuse for breaking the continuous residence?
The rules are that a term of imprisonment breaks a continuous qualifying period irrespective of whether the sentence is served in the UK or abroad. So if someone serves a sentence they need to have started a new qualifying period in the UK by 31/12/20 after they are released. It does not matter if they were prevented from returning to the UK by COVID-19 restrictions, if they did not return by 31/12/20 then they cannot apply to the EUSS / cannot qualify for settled status.
There are some useful video resources about EUSS and criminality from POMOC and New Europeans here: https://youtube.com/playlist?list=PLntgETBGNMnFow25eQO56JUICIHP-MvSu
Also, see Unlock (https://www.unlock.org.uk/), with a useful one page summary with helpful links: https://www.unlock.org.uk/wp-content/uploads/Settled-status-what-you-need-to-know-summary.pdf