Yeah, this is almost certainly a problem with the iPhone, either a limitation of the iMessage app itself or it being configured wrong by the user (who has probably never touched the settings).
Apple don't give a priority to it working well with other platforms as their users will often assume it's the other platform that doesn't work right and so won't want to switch (sad, but they have history there: it took 3 years and 2 class action suits for them to address the most severe problem, and that one had certainly driven a lot of switchers back to them and probably put others off). You could try talking to one of the iPhone owners and seeing whether iMessage has any settings related to this - we don't have an iPhone in my household so I can't check myself, and while 2 of my brothers use iPhones we mainly use WhatsApp anyway so don't encounter such issues.