I'm not so sure that Leica lenses are corrected into the infrared. Remember that an IR focus mark only applies to one wavelength, probably at one distance setting. With IR film you have a choice of filter and film combinations that result in different centre wavelengths. If you are focussing by eye, rather than by scale, through a red filter then the focus point may already be affected by wavelength.

I have read sources on the internet (Willem-Jan Markerink is one) that say that Leica do not put IR marks on their lenses nowadays because they could give a false sense of accuracy. Whether that is true or not, I share the view that the answer is to stop down.

Best,
Helen