Sorry to hear that ! Unfortunately I'm still on 7.17, can't test it on 8.x right now.
Here is a reformulation, where the row_number is computed once for each table, and then used in the mapColumn expression, hopefully this will fix the issue :
csv delimiter=',' data='A,B
Tom,1000
John,500
Adam,200'
| var_set name='current' value=0
| mapColumn name="row_number" expression={var name='current' | var_set name='current' value={math expression='add(value, 1)'}}
| var_set name='source_table' value={context}
| var_set name='rowCountSourceTable' value={var name='source_table' | rowCount}
| clear
| createTable rowCount=10
| var_set name='current' value=0
| mapColumn name="row_number" expression={var name='current' | var_set name='current' value={math expression='add(value, 1)'}}
| mapColumn name="resultA" expression={getCell column="row_number" | if condition={lt {var name='rowCountSourceTable'}} then={var_set name="current_row" value={context} | var name='source_table' | getCell column="A" row={var name="current_row"}} else={string 'n/a-' {context}}}
| mapColumn name="resultB" expression={getCell column="row_number" | if condition={lt {var name='rowCountSourceTable'}} then={var_set name="current_row" value={context} | var name='source_table' | getCell column="B" row={var name="current_row"}} else=0}
| alterColumn "resultB" type="number"
| pointseries x='resultA' y='resultB'
| plot defaultStyle={seriesStyle bars=0.75 horizontalBars=false} xaxis=true yaxis={axisConfig min=0}
| render