OTS11 — What happens to the existing transaction context on resume?

According to the OTS spec, there is no guaranteed way to get the previous
transaction back after calling Current::resume with another transaction.

Current::commit and Current::rollback only restore the previous transaction
context if the same thread started the transaction - otherwise the transaction
context is set to null. This means that you might have lost the previous
transaction that was replaced by "resume" ... or not, depending on where it was
created.

Is this desirable behaviour, considering that this suffers from the requirement
to keep track of all the transactions ever created by each thread? Why not just
restore the previous transaction context on commit and rollback?