Consider the following R block, which prints the occurrences of each
element in a list, including NAs:

#+begin_src R :session :results output :async
  table(c("ab","ab","c",NA,NA), useNA='always')
#+end_src

#+RESULTS:
:   ab    c <NA> 
:    2    1    2

Since Org 9.7, it instead prints:

#+RESULTS:
: ab    c <
: 2    1    2

The regression happens in commit:

e9c288dfaccc2960e5b6889e6aabea700ad4e05a

which made the prompt filtering more consistent between
`org-babel-comint-with-output' and `org-babel-comint-async-filter'.
However, it causes ob-R async session blocks to be over-aggressive in
removing the prompt.

Note that non-async ob-R blocks don't suffer from this problem,
because ob-R let-binds `comint-prompt-regexp' around
`org-babel-comint-with-output' (specifically, it adds a
beginning-of-line at the front of the regexp). However, I don't see a
good way to let-bind this around `org-babel-comint-async-filter'.

The best solution I could think of was to define a new buffer-local
variable, `org-babel-comint-prompt-regexp-override', which could be used
to override `comint-prompt-regexp' for the purpose of filtering. I
attach a patch with this solution.

Additionally, the regression causes causes misalignment of the output
due to removal of indentation. The fix for this is simpler, and
involves replacing a call of `org-trim' with `org-babel-chomp'.

I'm not sure if my patch is the best solution. But whatever solution
we arrive at, I would like to request that it be applied to bugfix
branch.