Citadel paints are water based while Tamiya paints and Tamiya thinner are alcohol based. I've always used water to thin my Citadel paints and haven't tried using Tamiya thinner with them, but since Tamiya thinner is mostly isopropyl (rubbing alcohol), it can dissolve acrylic paint, and only a tiny bit is needed to thin Tamiya acrylics. You might have used too much to thin your Citadel paints. I'd recommend using Lahmian medium or water to thin your paints when transferring to a dropper bottle.
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