UNION does not seem to work

[Frans Streur]

The following 2 SELECTS with a UNION does not seem to work. table 1 (Interacties) has 20 entries and table 2 (uren) has none. The field types are consistent and when I do the first select I get the 20 entries of table 1. When I add the UNION, I only get 10. I don't get any errors.

SELECT
`in`.`Datum` AS `Datum`,
`in`.`Interactietype` AS `Type`,
`in`.`Uren` AS `Uren`,
'J' AS `Direct`
FROM
`tbl_interacties` `in`
UNION SELECT
`ur`.`Datum` AS `Datum`,
`ut`.`Type` AS `Type`,
`ur`.`Uren` AS `Uren`,
`ut`.`Direct` AS `Direct`
FROM
(`tbl_activiteiten` `ur`
JOIN `tbl_acttypen` `ut`)
WHERE
(`ur`.`Seq_Acttype` = `ut`.`Seq`)

Thanks for the help.
Cheers,
Frans Streur
MySQL 5.7. on Windows7/CentOS7.3