I was working on some acroforms and wherever I needed a space I kept on getting a different letter (Ê). On debugging the problem seems to be in the TryGetCharCode function of OpenTypeFontSource

Version: 2019.3.917

Below is copy of function

public override bool TryGetCharCode(int unicode, out int charCode)
        {
            bool result = false;
            ushort glyphId;
            charCode = CMap.MISSING_GLYPH_ID;
            if (this.TryGetGlyphId(unicode, out glyphId))
            {
                ushort uCharCode = CMap.MISSING_GLYPH_ID;
                CMapTable table = this.CMap.GetCMapTable(3, 0);
                if (table != null)
                {
                    result = table.TryGetCharId(glyphId, out uCharCode);
                }

                table = this.CMap.GetCMapTable(1, 0);
                if (table != null)
                {
                    result = table.TryGetCharId(glyphId, out uCharCode);
                }

                charCode = uCharCode;

                return result;
            }

            return false;
        }

The font used has 2 cmap tables: one with platformid of 3 and encodingid of 1, the other with platformid 1 and encodingid 0. According to https://docs.microsoft.com/en-us/typography/opentype/spec/cmap platform id 3 and encoding 1 is correct for windows so not sure why the first call to getcmaptable looks for encodingid 0.

Second of all even if I change it to the following

CMapTable table = this.CMap.GetCMapTable(3, 1);

if there is a second cmap with platformid of 1 whatever the result of the call with regards to platformid 3, the result will be overridden

I can say that if I add some checks so that if the first call succeeds it doesn't attempt the 2nd I do get the expected behaviour in the pdf and get spaces

As a note the FontFamily property of the OpenTypeFontSource is "Arial"