The government is allowing companies to carry annual leave over for a period of 2 years if a business was able to cope.
https://www.gov.uk/government/news/rule ... g-covid-19
I'm not sure if it's fully proof in regards to yourself due to being off sick for other genuine reasons, but realistically seems mighty unfair.
However, if it's easier it's probably worth not going back to work right away and writing off the A/L. It's not like you can even book A/L as and when you need it.
I suggest the usual call to the union as normal.
If something a RM manager says sounds fishy, it probably is.