Message truncation issues on InspIRCd-v3 servers?

Hi Erc'ers,

When connecting to irc.rekt.network:6697 using `erc-tls', I've
encountered some message truncation issues during message sends:

It seems that Erc truncates 28 bytes after the 411th byte sent. I was
able to verify this using the following test:

1. Log into irc.rekt.network:6697 with 2 users (this is safe, hosts are
cloaked by default).
2. Start a QUERY session with the other user.
3. Execute (kill-new (mapconcat 'number-to-string (number-sequence 0
256) " ")) in the minibuffer.
4. Yank the number sequence from the kill ring into the query buffer.

The other user will receive: 0 1 2 3 4 5 6 7 8 9 10 11 12 13
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85
86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106
107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124
125 126 127 128 129 1 <NEWLINE>
137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154
155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172
173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190
191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208
209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226
227 228 229 230 231 232 233 234 235 236 237 238 239 <NEWLINE>
247 248 249 250 251 252 253 254 255 256

The range 130-136 is missing from the above listing. I suspect this is
being truncated by Erc, since I've confirmed with other users that other
clients do not exhibit this issue.

Note that this does not seem to happen on Libera. It may possibly be an ircd
issue. FWIW, the InspIRCd on irc.rekt.network advertises LINELEN=512 but
I am unsure whether this is relevant.

Any advice and/or debugging suggestions welcome.

Cheers,
-A.