We can use aggregate function ST_Collect() since MySQL 8.0.24.

It supports DISTINCT and OVER clause, and generates automagically a MULTIPOINT :

SELECT ALL
    SubjectID,
    ST_Collect(fix_geom),
    ST_AsText(ST_Collect(fix_geom)) -- MULTIPOINT((...),(...),...)
FROM ...
WHERE ...
GROUP BY 1

If you store points separately :

SELECT ALL
    SubjectID,
    ST_Collect(Point(latitude, longitude))
FROM ...
WHERE ...
GROUP BY 1

https://dev.mysql.com/doc/refman/8.0/en/spatial-aggregate-functions.html

We can use aggregate function ST_Collect() since MySQL 8.0.24.

It supports DISTINCT and OVER clause, and generates automagically a MULTIPOINT :

SELECT ALL
    SubjectID,
    ST_Collect(fix_geom),
    ST_AsText(ST_Collect(fix_geom)) -- MULTIPOINT((...),(...),...)
FROM ...
WHERE ...
GROUP BY 1

If you store points separately and/or need a GeoJSON :

SELECT ALL
    SubjectID,
    ST_Collect(Point(latitude, longitude)),
    ST_AsGeoJSON(ST_Collect(Point(latitude, longitude))) -- {"type": "MultiPoint", "coordinates": [...]}
FROM ...
WHERE ...
GROUP BY 1

https://dev.mysql.com/doc/refman/8.0/en/spatial-aggregate-functions.html