keepcolums is flacky

[JoranAngevaare]

Describe the bug
keepcolumns gives sometimes weird issues when it's run on not-yet stored data

To Reproduce

st = straxen.contexts.xenonnt(cmt_version='global_v6')
st.set_config({correction_name: correction_value})
df = st.get_df( ['018904', '018816'],
               'event_info',
               keep_columns='cs1',
               selection_str='(s1_area > 0) & (s2_area > 0)')
# Now this will fail also if I try it a second time
df = st.get_df( ['018904', '018816'],
               'event_info',
               keep_columns='cs1',
               selection_str='(s1_area > 0) & (s2_area > 0)')

Versions

Working on dali001.rcc.local with the following versions and installation paths:
python	v3.8.12	(default, Oct 12 2021, 13:49:34) [GCC 7.5.0]
strax	v1.1.5	/cvmfs/xenon.opensciencegrid.org/releases/nT/2022.01.4/anaconda/envs/XENONnT_2022.01.4/lib/python3.8/site-packages/strax
straxen	v1.2.6	/cvmfs/xenon.opensciencegrid.org/releases/nT/2022.01.4/anaconda/envs/XENONnT_2022.01.4/lib/python3.8/site-packages/straxen
cutax	v1.3.0	/dali/lgrandi/xenonnt/software/cutax/v1.3.0/cutax