Name one!
It would be quite an achievement.
SD
I know it *should* be quite an achievement, yet there have been more than one that I have come across. However, they are usually very beige boxes with little or no markings. i.e. not a "reputable" manuf.
I can tell you're doubting me from the posts here, but I shouldn't need to post reams of experience-related waffle purely to make a point. It exists, I have come across at least 3 in the last few years, but given that I deal with on average 2 (usually different) customer routers a month and have worked extensively in networking for decades, please just accept what i'm telling you - that a standalone AP is less hassle than an all-in-one.
To fulfil your point though, a Netgear DGFV338 with early release firmware exhibits this situation intermittently.
The cause is usually domestic CPE built to a price with cobbled-together open-source software from varying sources.