You do get a lot of maluses after conquering territory, and I think it might be possible for those to stack with other province modifiers to give -100% garrison size. To check the calculation, hover the mouse over the green garrison bar, and check the tooltip. You can also check holding modifiers (which appear as red icons on top of the picture of the tribe) and county modifiers (which appear as a row of icons below the picture of the tribe).
The "reset to zero" behaviour is weird, but might be explained by new modifiers being added or removed over time - eg. if you move your marshal around, or if you get some kind of negative effect (highway robbers? depopulation?).
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If you are rich, I would recommend holding the tribe personally while you construct a new castle in the province. Hand the new castle (and only the castle) to a new vassal, the hand that same vassal the whole county. This is the fastest way to convert tribal counties to feudal.
If you are semi-rich and the county follows the same religion as you, you could buy the necessary buildings to upgrade the tribe to a castle. Slow, but less expensive that building a new castle, and the upgraded castle will probably be better than a newly-constructed one, and it offers a roundabout way to construct castle buildings with prestige rather than gold.
If you are not rich, hand the tribal county to a vassal, because it's just not worthwhile to hold it for yourself. It's probably beat to choose a previously-unlanded character of your religion and culture (he will become tribal and will veeeeery slooowly upgrade to feudal), but other tactics might work in niche situations.