user47827: BIP 8 states that:

During the MUST_SIGNAL phase, if (2016 - threshold) blocks in the
retarget period have already failed to signal, any further blocks that
fail to signal are invalid.

Why doesn't it enforce 100 percent signaling during the MUST_SIGNAL phase rather than merely the threshold?

user47827: nickler answered this on IRC.

In a mixed network, the rules can become active, but in two separate chains because lot=true nodes will reject blocks from the lot=false chain which doesn't require 100% signaling.

The MUST_SIGNAL phase (when lot=true) overlaps with the last two weeks (2016 blocks) of the STARTED phase (when lot=false). In the case that the threshold of miners signaling is met (but not 100 percent of miners signaling) during these two weeks you don't want to create a chain split between lot=true and lot=false nodes for no reason. If MUST_SIGNAL enforced 100 percent signaling and there wasn't 100 percent signaling during this period you would get a chain split between lot=true and lot=false nodes.

47827: <p><a href="https://github.com/bitcoin/bips/blob/master/bip-0008.mediawiki#new-consensus-rules" rel="nofollow noreferrer">BIP 8</a> states that:</p>
<blockquote>
<p>During the MUST_SIGNAL phase, if (2016 - threshold) blocks in the
retarget period have already failed to signal, any further blocks that
fail to signal are invalid.</p>
</blockquote>
<p>Why doesn't it enforce 100 percent signaling during the MUST_SIGNAL phase rather than merely the threshold?</p>
 47827: <p>nickler answered this on IRC.</p>
<blockquote>
<p>In a mixed network, the rules can become active, but in two separate chains because lot=true nodes will reject blocks from the lot=false chain which doesn't require 100% signaling.</p>
</blockquote>
<p>The MUST_SIGNAL <a href="https://github.com/bitcoin/bips/blob/master/bip-0008.mediawiki#state-transitions" rel="nofollow noreferrer">phase</a> (when lot=true) overlaps with the last two weeks (2016 blocks) of the STARTED phase (when lot=false). In the case that the threshold of miners signaling is met (but not 100 percent of miners signaling) during these two weeks you don't want to create a chain split between lot=true and lot=false nodes for no reason. If MUST_SIGNAL enforced 100 percent signaling and there wasn't 100 percent signaling during this period you would get a chain split between lot=true and lot=false nodes.</p>
