In my setup, i have an index of names:

\defineregister[Nome][...]

The indexing of names can be toggled with a mode: --mode=AddNames

The setup is something like this:

\startmode[AddNames]
  \Name[key]{term}\xmlflush{#1}
\stopmode
\startnotmode[AddNames]
  \xmlflush{#1}
\stopnotmode

I noticed that \Name[key]{term} adds some space, leading to a different typesetting of the paragraph in rare cases (especially in a paragraph full of names).

Can \Name[key]{term} add some space, or should i look for a different bug in my setup?

Sorry, no MWE; i tried, but i could not reproduce the problem in a minimal example.

This has been a long-standing issue with registers in straight (non-XML) code. I always follow a \index{} with a % to avoid it. I suspect that will help here as well, splitting onto two lines, as

\Name[key]{term}%
\xmlflush{#1}

or perhaps

\Name[key]{term}\letterpercent
\xmlflush{#1}

Have you tried this?

(I cannot construct an MWE at this point either.)

--
Rik