Has anyone a solution / hint as how to get the linespacing correct when using columns ?

The first page works as expected, 
but all the following pages have an unexpected mis-alignment  in the first column





The document itself is plain vanilla:


\starttext
\startlines
Testing 1971-04-26
\startcolumns[n=3]

+ 17271 \rightarrow\  2018-08-08
+ 17300 \rightarrow\  2018-09-06
+ 17371 \rightarrow\  2018-11-16

…


+ 39900 \rightarrow\  2080-07-22
+ 39993 \rightarrow\  2080-10-23
+ 40000 \rightarrow\  2080-10-30
\stopcolumns
\stoplines
\stoptext